DER reading/writing for X509, X509Req, & RSA

[afcady]

This PR incorporates code from myself, @newsham, and @shak-mar to provide DER (de)serialization for different OpenSSL types.

Also included is support for adding X509v3 extensions (e.g., subjectAltName) to X509Reqs.

These branches are merged in this PR:

• PR add DER reader and writer for X509 #39 from @newsham
• PR Add DER encoding and decoding support for RSA public keys #46 from @shak-mar

Also relevant:

• Bug DER support? #21

[vshabanov]

I'm new maintainer of HsOpenSSL. Pull request was merged into https://github.com/vshabanov/HsOpenSSL and released on Hackage. In future please send pull requests to the new repo.

[afcady]

Thanks @vshabanov !